Cantilever sensors have attracted considerable attention over the last decade because of their potential as a highly sensitive sensor platform for high throughput and multiplexed detection of proteins, nucleic acids and other molecules.
Biological specificity in detection is typically achieved by immobilizing selective receptors or probe molecules on one side of the cantilever using surface functionalization processes.
Biomolecular interaction between the immobilized receptors or probes and a coupling molecule in a body fluid causes a measurable bending of the cantilever. This nanoscale deflection is caused by a variation in the cantilever surface stress due to the biomolecular interaction and can be measured by optical or electrical means, thereby reporting on the presence of specific molecules and their quantitation in the body fluid.
The cantilever bending is a function of the number of molecules bound to the probe molecules on its surface.
Biosensing technologies based on cantilever arrays have the potential of satisfying the need for multi-target detection with high sensitivity and selectivity using very small volumes of sample.
When cantilevers are made softer with very small force constants, they can measure forces and stresses with extremely high sensitivity.
The very small force constant (typically less than 0.01 N/m) of a cantilever allows detection of surface stress variation due to the adsorption (or specific surface-receptor interaction) of molecules.
Known cantilever sensors are typically made of rectangular silicon, or polyamide polymer materials coated with a layer of gold on one side (the top surface) and with non-reactive layer of molecules such as PEG-silane on the other side (the bottom surface). The PEG-silane coating of the bottom surface is called passivation layer. The purpose of passivation of the bottom surface is to help avoiding unwanted functionalization of the bottom surface with receptors or probe molecules, consequently preventing probe molecule (ligand) adsorption that would alter sensing results. Receptors or probe molecules are typically immobilized on the cantilever top gold surface using, for example, alkanethiol chemistry.
The need for mass-produced, miniature microcantilever arrays having unprecedented sensitivity for label-free biodetection applications, such as toxin, protein, drugs and antibody detection, DNA hybridization, selective detection of pathogens etc. is significant. However, improvements in cantilever sensitivity and selectivity are far from finished and there is still a long felt need in the sector of label-free molecular detection.
Passivating of the underside of the cantilever to prevent unwanted ligand adsorption is lengthy and often requires tedious optimization. For example, on average more than 60 minutes is necessary to passivate a single micro-cantilever array with PEG-silane. Moreover, the detailed investigation of Si surface passivation shows that the process of cantilever underside coating to prevent unwanted adsorptions is far from complete and therefore needs further optimization.
Therefore, it would be desirable to provide cantilever sensors without the need of a passivation layer but, at the same time, having the same or even an enhanced selectivity and sensitivity with respect to the known passivated cantilevers.